Process for pretreating decarbonized steel to be directly enamelled

ABSTRACT

PROCESS FOR PRETREATING DECARBONIZED STEEL TO BE DIRECTLY ENAMELLED AND IN PARTICULAR FLAT PIECES OF STEEL TO BE USED AS LININGS FOR HEARTS AND REFRIGERATORS IN WHICH FOLLOWING THE PURIFICATION AND ETCHING OF THE STEEL WORK PIECE, AN ADHESIVE COATING LAYER IS APPLIED COMPRISING THE STEPS OF SUBJECTING SUCH STEEL WORK PIECE TO A BOILING DEGREASING, THEN TO AN ELECTROLYTIC ALKALINE DEGREASING, A RINSING, AN ELECTROLYTIC ETCHING USING A SULFURIC ACID ELECTROLYTE, A RINSING AN ELECTROLYTIC APPLICATION OF AN ADHESIVE METAL COATING, A RINSING, NEUTRALIZATION AND A DRYING.

United States Patent 3,725,218 PROCESS FOR PRETREATING DECARBONIZED STEEL TO BE DIRECTLY ENAMELLED Christian Friedel, Suhl, and Manfred Faethe, Dresden, Germany, assignors to Veb Elektrogeratewerk Sub], Sub], Germany No Drawing. Filed Feb. 11, 1971, Ser. No. 114,705 Int. Cl. C23h 1/04; (323E 17/00 US. Cl. 204-34 8 Claims ABSTRACT OF THE DISCLOSURE Process for pretreating decarbonized steel to be directly enamelled and in particular fiat pieces of steel to be used as linings for hearths and refrigerators in which following the purification and etching of the steel work piece, an adhesive coating layer is applied comprising the steps of subjecting such steel work piece to a boiling degreasing, then to an electrolytic alkaline degreasing, a rinsing, an electrolytic etching using a sulfuric acid electrolyte, a rinsing an electrolytic application of an adhesive metal coating, a rinsing, neutralization and a drying.

This invention relates to a process for pretreating decarbonized steel prior to the direct enamelling of the same and more particularly relates to a process for pretreating decarbonized fiat pieces of steel as for instance the lining sheets for forges, hearths, refrigerators and the like in which after the purification or refining and etching of the steel work piece an adhesive met-a1 coating is applied thereto.

A process is already known for pretreating iron articles having a carbon content of at most 0.15% for improving the adhesiveness of a subsequently applied enamel coating in accordance with which the iron article or work piece is subjected to a treatment in an electrolytic purifying or refining bath using a current density of 1M a./cm. for a period of 30 seconds. After rinsing, the electrolytically purified article is electrolytically etched with sulfuric acid of 50 B. for a period of 30 seconds using a current density of 5M a./cm. There is then applied onto the water rinsed iron work piece, an adhesive metal coating layer utilizing a chemical reduction solution composed of 30 g./l. NiCl2, 5H O, l0 g./l. sodium acetate and g./l. sodium hypophosphate having a pH of 4-6 at a bath temperature of 80 C. over a treatment period of 30 seconds. Up until the subsequent enamelling, the thusly treated iron work piece for protection of the adhesive metal coating layer is required to be covered over with a protective coating.

The disadvantage of this process is that it is not possible or is only possible with great difiiculty to adapt the same to a continuous process. This is because the process step where the adhesive metal layer is reduced is very time consuming in relation to the other steps and the chemical method which is involved does not permit or allow for the shortening of this step.

A further disadvantage of applying the adhesive metal coating layer by chemical means lies in that the bath temperature required for the nickel exchange amounts to at least 80 C. necessitating a high energy consumption. Further, the chemical application of adhesive metal coatings does not make it possible to keep the back or reverse side of the iron work piece which is not to be provided with enamel, free of the adhesive metal coating. This situation is not only economically disadvantageous but also has technical disadvantages. Because of the finely distributed adhesive metal precipitate formed on the unprotected reverse side of the work piece, there arises through local element formation, an increased susceptibility to corrosion. The chemical adhesive metal layering process is further associated with high costs for chemicals, short periods of stability of the baths and the need to provide regeneration equipment, etc.

There has also already been proposed a process for improving the adhesiveness of an enamel coating layer on normal steel surfaces in which process the nickel coating is electrolytically carried out. In carrying out this process for pretreating normal steel, it is an essential condition that the steps of nickeling, annealing to form an oxide layer, and acid etching be carried out in a specified order. The process steps which are additionally required can be carried out in an optional order, i.e., following or intermediate the previously mentioned steps. That is this process can be carried out using the sequence of processing steps of normalizing, etching, nickelizing, oxidizing, annealing and etching. The decarbonization then takes place with the nickelizing, annealing and etchig steps following one another in sequence. This process is specifically adapted for the pretreatment of normal steel and is therefore not suitable for the pretreatment of decarbonized steel. The electrolytic nickel coating step as carried out in this process takes place in a nickel cyanide or in a nickel ammonium cyanide bath or in a dilute solution of a somewhat altered Watts bath. The electrolyte consists in this case of at least nickel sulfate and nickel chloride in aqueous solution. The current density as used in the electrolytic nickel coating step amounts to- 2.1 to 3.2 a./dm.

The use of cyanide containing electrolyte baths has the disadvantage that they are exceedingly toxic. The use of Watts baths is further very uneconomical in that such baths require as a rule at least two chemical substances. The use of these multiple materials results in higher costs both for them and for the chemical alkalies required and as well involve protracted bath preparations and high storage costs.

The use of Watts baths further involves high temperatures and this also adds to the costs, in this case for the increased energy required for providing the high temperatures. Further as in the Watts baths, there are present more or less free chlorine ions, enamelling defects in the form of blisters often arise.

It is an object of the invention to provide a method for substantially reducing the time and also the costs of applying adhesive metal coating layers to metal work pieces prior to the enamelling thereof.

It is another object of the invention to provide a method for applying adhesive metal coating layers onto metal work pieces utilizing non-toxic baths.

Still another object of the invention is to provide such a method particularly adapted for use with decarbonized steel work pieces.

Further objects, additional advantages and other novel features of the present invention will become apparent from the appended claims and from the ensuing detailed discussion and description of the invention.

In accordance with the invention the steel work pieces to be treated prior to the enamelling thereof are first subjected to a boiling degreasing or scouring, followed by an electrolytic degreasing or scouring, and after a rinsing to an electrolytic etching in sulfuric acid. After a further rinsing of the etched work piece, the electrolytic adhesive layer formation is carried out and is followed by the conventional rinsing, neutralization and drying steps.

Thus in accordance with the invention the work piece and specifically the decarbonized steel work piece is subjected to a boiling degreasing, then to an electrolytic alkaline degreasing, rinsing, electrolytic etching in sulfuric acid, rinsing, electrolytic application of an adhesive metal coating layer, rinsing, neutralization and drying.

In accordance with a preferred feature of the process of the invention, the electrolytic boiling alkaline degreasing is carried out using a current density of about a./ dm.

In accordance with a further preferred feature of the process of the invention the electrolytic etching is carried out using a dilute sulfuric acid solution as electrolyte and a current density of about 12 a./dm.

Still a further embodiment of the invention lies in carrying out the electrolytic adhesive metal coating step in an ammonium Ni(II) sulfate bath at a temperature of about 20 C. and a current density of 1.0 to 3.0 a./dm.

The advantage of the process of the invention lie in the decreased costs for chemicals and also for alkalies required to be used in the adhesive metal layer deposition and in the etching steps. Also the stability of the baths involved is increased. It is also important that only nontoxic electrolyte solutions are used. Still another advantage is that the formation of the adhesive metal layers take place at room temperature whereby energy costs are reduced. The electrolyte used in the nickel coating step is composed of only one substance which considerably simplifies matters. The process of the invention further makes possible the application of the adhesive metal layer to one side only of the work piece whereby in addition to the attendant economic advantages, a lowered susceptibility to corrosion of the enamel free reverse side of the work piece is achieved. Still further through the use of chlorine free nickel electrolytes the waste or defective pieces obtained in the enamelling are significantly lowered.

By means of the process of the invention the number of the hitherto conventional process steps as well as the duration thereof, in particular of the process step for achieving the adhesive metal coating are decreased. From the above the advantages and benefits of the process of the invention can be appreciated.

The following example illustrates the present invention but is not to be construed as limiting.

EXAMPLE A steel plate having a content of 0.005% carbon was treated by the process of the invention. The steel plate was treated so that only one side thereof was prepared for, i.e., pretreated for subsequent enamelling. The treatment of one side was realized by arranging the opposite electrodes with respect to one another so that the side of the steel sheet to be subsequently enamelled faced in wardly and toward the other electrode. Of course it is also possible in accordance with the invention for both sides of the steel plate to be simultaneously pretreated so as to prepare both sides for subsequent enamelling.

The treatment took place in accordance with the fol lowing sequence of steps.

Following a three minute degreasing treatment in a boiling 4% Siliron KZ bath, an electrolytic alkaline degreasing treatment was carried out in a 4% boiling Siliron KE solution within one minute and at a current density of 5 a./dm. In the electrolytic degreasing, the steel plate to be treated served as the anode. A rinsing with water was thereafter carried out at a temperature of 50 C. within a five minute period. This was followed by a one minute cold water rinsing step. The etching step was conducted using a sulfuric acid which had a temperature of 75 C. and was carried out over a period of 0.5 minute, the necessary metaletching being achieved in that time. The current density amounted to 12 a./dm. for the etching step and again the work piece constituted the anode. A five minute cold water rinsing step was then carried out afterwhich the electrolytic nickelizing took place. The work piece was now the cathode. As electrolyte, there was used a solution containing 50 g. ammonium Ni(II) sulfate per liter. The electrolytic adhesive metal coating step was carried out at room temperature using a current density of 1.5 a./dm. in a constantly stirred bath within about one minute. After the electrolytic adhesive metal coating had been completed, a one minute cold water rinsing was carried out and was followed by the passivating and drying steps.

What is claimed is:

1. Process for the pretreatment of decarbonized steel prior to the enamelling thereof which comprises subjecting a decarbonized steel metal work piece to the following steps in the sequence indicated: (1) boiling degreasing, (2) electrolytic alkaline degreasing, (3) rinsing of the degreasing steel work piece, (4) electrolytic etching in sulfuric acid, (5) rinsing of the etched work piece, (6) electrolytic adhesive metal coating in an ammonium Ni(II) sulfate bath of concentration about 50 g./l. and temperature of 20 C. and at a current density of 1.0-3.0 a./dm. and for a period of about 1 minute thereby to deposit finely divided nickel on the work piece, (7) rinsing, (8) neutralizing and (9) drying.

2. Process according to claim 1 wherein the electrolytic alkaline degreasing step (step 2) is carried out at a current density of about 5 a./dm.

3. Process according to claim 1 wherein the electrolytic etching (step 4) is carried out with a dilute sulfuric acid solution and using a current density of about 12 a./dm.

4. Process according to claim 3 wherein the metal work piece is used as the anode in the electrolytic etching step.

5. Process according to claim 1 wherein the metal work piece is utilized as the cathode in the electrolytic adhesive metal coating step.

6. Process according to claim 1 wherein said current density is 1.5 a./dm.

7. Process according to claim 1 wherein said electrolytic etching and coating are carried out on one side only of said metal work piece.

8. Process according to claim 1 wherein said electrolytic etching and coating are carried out simultaneously on both sides of said metal work piece.

References Cited UNITED STATES PATENTS 2,819,207 1/ 8 Shepard 20434 2,748,066 5 195 6 Whitehouse et al 20434 3,317,340 5/ 1967 Ziehr et a1 20434 OTHER REFERENCES The Anodic Etch in Preparation for Plating, by F. G. Brune et al., Plating, September 1955, pp. 1127-1132.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R. 20438 C 

